1. Field of the Invention
This invention pertains to electric arc welding guns, and more particularly to the contact tips and diffusers of MIG welding guns.
2. Description of the Prior Art
It is well known to incorporate a contact tip into MIG welding guns. The contact tip guides the weld wire for the last short distance from the gun to the workpiece. In addition to guiding the weld wire, the contact tip also performs the function of conducting electric current to the weld wire. In turn, the electric current must be conducted to the contact tip, which is normally done through an interface with a diffuser.
The dual functions of guiding the weld wire and conducting electric current to it impose severe thermal stresses on the contact tip. Not only is it subjected to very high temperatures from the welding arc, but also the transfer of high current through it tends to produce resistance heating in it. To minimize any temperature increases because of resistance heating due to electrical conduction from the diffuser to the contact tip, it is very desirable that the surface areas in contact between the diffuser and the contact tip be as large as possible. Another advantage of large surface areas in contact is that any heat buildup in the contact tip can be readily conducted away to the diffuser.
Although ongoing improvements have been made to contact tips, under some operating conditions they nevertheless have relatively short service lives. When a contact tip fails, it is important that it be easily and quickly replaced. The consumable nature of contact tips also makes it important that they be as inexpensive as possible consistent with proper performance.
To achieve rapid contact tip replacement, it is known to use a contact tip-diffuser combination in which the contact tip is removed from and replaced onto the diffuser with approximately a single turn of the contact tip. For example, U.S. Pat. No. 5,760,373 describes a prior quick release welding gun contact tip that is disengageable from the diffuser with a single turn. The U.S. Pat. No. 5,760,373 patent also describes a large area of contact between the diffuser and the contact tip.
Despite continued improvements to contact tips and diffusers, further developments are needed.
In accordance with the present invention, a welding gun contact tip with rolled threads provides both economy and high performance. This is accomplished by a threaded connection between the contact tip and a diffuser that uses custom threads having a pitch diameter that is disproportionately large relative to the size of the contact tip.
The contact tip has upstream and downstream ends and a long hole for guiding a weld wire. Adjacent the downstream end is a cylindrical section. The outer diameter of the cylindrical section can vary to suit welding gun space and functional requirements. However, for many MIG welding guns, a preferred outer diameter for the cylindrical section is approximately 0.350 inches. The upstream end of the contact tip is formed as a tapered section with an exterior frusto-conical surface converging toward the upstream end.
Between the contact tip frusto-conical surface and the cylindrical section is a threaded section. The threaded section is comprised of threads that are rolled from the cylindrical section. The threads are rolled such that their major or crest diameter is larger than the diameter of the cylindrical section, and the threads minor or root diameter is less than the diameter of the cylindrical section. For example, threads rolled from a cylindrical section having the 0.350 inches diameter may have a major diameter of approximately 0.375 inches and a minor diameter of approximately 0.320 inches. Preferably, the threads are stub acme threads.
To facilitate the rolling process, the threads have a special pitch that is finer than the standard stub acme thread corresponding to the thread major diameter. Specifically, for stub acme threads having a major diameter of 0.375 inches, the threads may have a pitch of 14 threads per inch, instead of the standard pitch of 12 threads per inch.
The diffuser has front and back ends and a bore therebetween. The diffuser bore at the back end has conventional threads for connecting with another component of the welding gun. The diffuser bore at the front end has the same custom threads as the contact tip. Adjacent the diffuser custom threads is an interior frusto-conical surface of the same size and shape as the contact tip frusto-conical surface.
The contact tip is assembled to the diffuser by inserting the contact tip upstream end into the diffuser bore at the diffuser front end until the threads of the contact tip abut the threads of the diffuser. The contact tip is turned slightly, if necessary, until the threads of the two components mate. The contact tip is then turned to advance it into the diffuser bore. The threads and frusto-conical surfaces of the two components are so dimensioned that the contact tip frusto-conical surface engages the diffuser frusto-conical surface after about a single turn of the contact tip threads onto the diffuser threads.
When the contact tip is fully assembled on the diffuser, there are two large areas of contact between them for conduction of electric current. The two frusto-conical surfaces present a first large contact area. Second, the increased major diameter of the custom stub acme threads provide a greater contact area between them than would be possible with a contact tip having a threaded section with the threads major diameter equal to the cylindrical section outer diameter. Moreover, only about a single turn is required to remove and replace a worn contact tip.
Other advantages, benefits, and features of the present invention will become apparent to those skilled in the art upon reading the detailed description of the invention.